The present invention relates to radio systems and, more particularly, to a method and apparatus for receiving radio signals with the aid of antenna beams.
The quality of a received radio signal is affected by many natural phenomena.
One of these phenomena is time dispersion, which is caused by a signal on its way from a transmitter being reflected by obstacles at different locations in the propagation path before reaching the receiver. The signal will arrive at the receiver at different time delays, due to the different propagation paths along which the signal travel. With the introduction of digital coded data in radio systems, time dispersed signals can be successfully restored. It is well known to a person skilled in this art to use a RAKE-receiver or an equalizer to restore a time dispersed signal.
Another phenomenon, called fast fading or Raleigh fading, is caused by the signal being scattered on its way from the transmitter to the receiver by objects in a near distance from the transmitter or receiver. Thus, different signal versions that are slightly shifted in phase in relation to each other are received. In area spots where the phase differences are unfavorable, the sum of the received versions of the signal is very low, even close to zero. This results in a fading dip wherein the received signal virtually disappears. Fading dips occur frequently with a distance in the same order as the wavelength. For the 900 megaherz radio band the distance between two fading dips may be in the order of 15-20 cm. In case of a moving transmitter or receiver, the time that elapses between two successive fading dips as a result of fast fading depends on both the carrier frequency of the signal and the speed of the transmitter in relation to the receiver.
One well known method of combatting fading is to provide the radio receiving station with an antenna diversity system. The system comprises two or more receiving antennas separated either spatially or by orthogonal polarization directions, or by a combination thereof. As a result fading of the signals received by each antenna are less correlated, thus decreasing the possibility of both antennas being exposed to a fading dip at one and the same time. To enable radio reception of both signals received by the antenna diversity arrangement the radio receiver station is provided with separate receiver branches for each receiving antenna.
A third phenomenon troublesome to radio transmission is that of interference. An interfering signal can be characterized as any undesired signal received on the same channel as the desired signal. For military radio systems the most important interference to combat is jamming, i.e. intentional disturbance by the enemy. For cellular radio systems the interference problem is closely related to the capacity demand for communication. As the radio spectrum is a scarce resource, a radio frequency band given to a cellular operator must be used efficiently. For this reason the operators service area is divided into cells and radio channels used in one cell are reused in cells that have a minimum number of cells in-between. Because of the popularity of mobile phones, the demand for traffic capacity has grown rapidly. One way of handling the capacity demand is to decrease the size of the cells, thus enabling closer reuse of the channels per area unit and thereby raising the communication capacity of a given area while still preserving the frequency-reuse-factor.
In areas where the capacity demand is high, such as in city centres and railway stations, it is often difficult to find sites for base stations. An available place for a base station may have the form of a wall on which it can be hung. In the case of sites of this nature, it is important that the radio base station is small and demands less power. The size of the radio base station is related to the power consumption, since power necessitates cooling and cooling necessitates space. The appearance of the installation is also important, for instance with respect to obtaining permission from the authorities to use a new radio base station site.
Because of the increasing popularity of cellular systems there is a need to find new ways to combat interference, and thereby also enable higher traffic capacity. For this reason the use of adaptive antennas in radio base stations in cellular systems has been met with great interest, though not yet implemented in any commercial system. An adaptive antenna is commonly comprised of an antenna array connected to beam forming means. The adaptive antenna forms a set of antenna beams which each covers a narrow predefined space area and which together cover a wide predefined area omnidirectionally or within a sector. A signal sent from a mobile transmitter is received by each of the antenna beams, each version of the signal being separately received and thereby the angular information being maintained. The angular information is inherent in the phase difference between the different versions of the signal. An estimation of the direction to the signal source is made on the basis of the demodulated versions of the received signal. This estimated parameter is also called DOA, direction of arrival.
To enable the estimation of the DOA, signals received by each beam must be received separately by corresponding radio receiver branches.
The DOA-estimation is used for the selection of one or more antenna beams, or for directing of a narrow steerable beam, for transmission in the downlink to the mobile of interest. Transmission in the chosen beam is directed to the mobile station whereby mobiles that use the same channel in other directions will be less exposed to interference. Downlink interference is thus combated by means of the adaptive antenna technique.
One method of contending with fading and with the results of interference is to cause a radio channel frequently to change its carrier frequency. This method is called frequency hopping and is used with some success in the GSM-system. Patent publication U.S. Ser. No. 08/768319, adresses in respect of frequency hopping systems a problem that resides in the coherence bandwidth being wider than the frequency bandwidth available for operation. This implies that carrier frequencies used for frequency hopping have a correlated fading. Thus the purpose of frequency hopping to combat fading can not be achieved. The solution proposed in U.S. Ser. No. 08/768319 invovles producing a smaller coherence bandwidth by introducing an artificial delay spread. One way of producing the artificial delay spread is to receive a signal on two antennas, delay the signal received by a first of the antennas and then combine the delayed signal with the signal from the second of the antennas. The two combined signals are then fed to one receiver.
U.S. patent specification, U.S. Pat. No. 5563610, addresses the use of a multi-beamforming antenna for the purpose of gaining antenna diversity based on the different beams being very narrow and covering disjunct areas. This is called angular diversity and results in the signals received in separate beams being uncorrelated. For this purpose U.S. Pat. No. 5563610 teaches a receiving system in which branches from each antenna beam are distributed into two groups. In one group signals are delayed in relation to one another and then combined. Two combined signals, each derived from a corresponding of the two groups, are thus obtained and then fed to a conventional CDMA-receiver.
In this receiver the angular information is lost after the signals have been combined. It is thus impossible to make a DOA-estimation and by means of beamforming combat downlink interference.
The present invention addresses a problem arising when both an enabling of accurate DOA-estimation and antenna diversity shall be provided in a radio receiver comprising a limited number of radio receiver branches. The limited number of radio receiver branches results in a trade-off between the accuracy of the DOA estimation and the performance of the antenna diversity reception. If all the receiver branches are used in the DOA estimation process, the lack of protection against fading will lower the performance of the DOA estimation. If, on the other hand, the diversity gain is to be maintained by separate reception of less correlated signals, the number of beams that can be received separately will be reduced and thus also the accuracy of the DOA estimation.
Another problem is to produce a radio base station comprising a radio receiver system that is small, has low power consumption and has antenna diversity as well as means for estimating DOA. It will be remembered that receiver branches need space and are power consuming.
The object of the present invention is to make reception possible both by antenna diversity and by antenna beams to enable an accurate estimation of DOA, and to combat fading in a receiver that includes only a moderate number of receiver branches and thus achieve the aim of providing a radio station that is both compact and requires less power.
The essence of the present invention is the introduction of an artificial time dispersion in a set of signals received by antenna diversity and by different antenna beams. Sets of signals received by different antenna assemblies are delayed relative to one another and signals that derive from beams covering the same space area are combined. For each of the beams in the first antenna assembly there is a beam in each of the other antenna assemblies that covers the same space area. In this way the angular information is maintained. Each combined signal is then radio received in a joint radio receiver. A DOA-estimation can be calculated on the basis of radio received signals derived from all beams. Both the natural and artificial time dispersion of the radio received signals can be restored in an equalizer or a Rake-receiver. By the inventive combination of signals the energy from each of the combined signals is maintained until the signals reach the equalizer or Rake-receiver. The energies from the different time dispersed signals are merged together in the equalizer or in the Rake-receiver. If the energy of one of the combined signals is low temporarily due to a fading dip at the corresponding receiving antenna the energy of the signal received by the other antenna will compensate for the fading dip.
More precisely, the present invention solves the aforementioned problems by means of a method in which signals are received by at least two antenna assemblies that are separated to achieve antenna diversity, i.e. the antenna assemblies are separated spatially or by different polarization directions. Each of the antenna-assemblies generates a set of antenna beams. The antenna assemblies are constructed so as to generate mutually corresponding sets of antenna beams, i.e. the beams have corresponding angular coverages and a particular area is covered by two beams, one from each of the antenna assemblies. Signals received by separate antenna assemblies in corresponding antenna beams are then mutually combined after having been delayed in relation to one another. An artificial multipath propagation is thus created in respect of the combined signal. The combined signal is then fed to one radio receiver branch for frequency transformation from RF to a lower frequency and demodulation, whereupon the artificial time dispersion can be restored by digital signal processing in an equalizer or a RAKE-receiver for instance. A DOA-estimation can be calculated on the basis of the outputs from several radio receiver branches to which signals are fed from separate beams.
The present invention is also related to a radio receiver system which solves the aforementioned problems. The radio receiver system comprises at least two antenna assemblies which are mutually separated to achieve antenna diversity. Each of the antenna assemblies generates a set of antenna beams, where each beam covers a narrow space area and the beams together cover an specific area omnidirectionally or within whithin a sector. The different sets of beams correspond to each other, and one space area is covered by a beam from each of the antenna assemblies. Delay elements are connected to all but one of the antenna assemblies. The delay elements delay signals received by a corresponding antenna assembly. The delay is given a separate value for each antenna assembly. A number of combiners are connected to the delay elements and also to that antenna assembly which is without delay element. Each of the combiners receives from each of the antenna assemblies signals from corresponding beams. Each combiner output is connected to a corresponding receiver branch.
The invention constitutes an improvement in the known art by virtue of the fact that one radio receiver branch can be fed with signals from several antenna assemblies, whereafter the signals can be restored. Thus the required number of radio receiver branches to achieve both antenna diversity gain and to enable the calculation of an accurate DOA estimation is limited to correspond to the number of beams in the set of antenna beams. This enable both the size of the radio receiver and its power consumption to be reduced.
A further improvement is found in respect of sites in which a base station comprising the inventive radio receiver is placed on the ground and the antenna assemblies are mounted on a mast. The weight of the cables connecting the base station with the antenna assemblies is an important factor in respect of mast dimensions. The number of cables connecting the base station to the antenna assemblies can be reduced by coupling the combiners close to the antenna assemblies. Thereby the weight of the cables is reduced which will allow a mast to have smaller dimensions and therewith lower the cost of the mast as well as the cables.